The present invention relates generally to lithography in semiconductor processing, and more particularly to an apparatus and method for immersion lithography.
Immersion optics has long been practiced as oil immersion microscopy. Typically, a few drops of oil envelop a high magnification microscope objective and a specimen. The effect is to increase the numerical aperture of the objective. Typically, since the stage and specimen are relatively static, the oil stays in place.
In semiconductor lithography, liquid immersion lithography may produce an effective numerical aperture greater than 1 when a liquid with a refractive index of greater than 1 is introduced between the projection lens and the wafer that is being patterned. A numerical aperture of greater than 1 is achievable because the larger final lens that faces the wafer, projects light into such a liquid rather than air. When a lens projects light into air, internal reflections may undesirably result at the lens-air interface.
Liquid immersion lithography can be useful with 193 nm wavelength exposure light currently in use. It is even more likely to find use with 157 nm, or shorter, wavelength exposure light. At these wavelengths, protective mask pellicles are available. Proven photoresists are also available. The required lenses are not larger or more complicated. An appropriate liquid is required. Since the sequential exposure of multiple sites on a semiconductor wafer using a step-and-repeat photolithography tool is a dynamic event, the stage for the semiconductor wafer will need to handle and contain a flowing liquid.
There are at least two styles of stages. In the bath mode, the wafer surface is always totally immersed. In the shower mode, sufficient fresh fluid flows to immerse, or fill, the space between the lens and the wafer but not necessarily the entire wafer surface at one time. In both styles, the proper handling of the escaping fluid is of great importance. For example, the moving fluid may stir up particulate contamination, a major source of which is at the beveled edge of the wafer. Since this edge is typically formed by grounding the wafer to a rounded shape, it includes a relatively rough surface. Because the processing operation used to grind the wafer is poorly controlled, portions of the rough surface itself and residual materials on and near the rough surface, may easily delaminate in particulate form. The redeposition of these particulate materials on the critical face of the semiconductor wafer is to be avoided.
It would therefore be desirable to provide a suitable immersion lithography system for semiconductor manufacturing. It would also be desirable in the art of immersion lithography, to control the direction of liquid flow, thereby reducing particulate contamination.